The present invention relates to a method of and an apparatus for inspecting a streak on a sheet product with ease, the streak being low in contrast, narrow in width and long in a direction along which the sheet product is to be transported.
For example, in the manufacturing line for producing sheet products, such as a paper, a film, a steel plate or the like, it is necessary to inspect whether the sheet product has surface defects formed thereon, such as lines, scratches, irregularities, dirty marks (blots), alien substances etc.
In order to inspect such surface defects, it is general to take respective pictures of the sheet product by means of a xe2x80x9cline sensorxe2x80x9d camera arranged above the materials on transportation. Then, video signals generated from the line sensor camera are processed for images in order to emphasize the defects.
As to this image processing, Japanese Unexamined Patent Publication (Kokai) No. 7-225196 discloses a method of detecting fine (microscopic) defects. In the method, the video signals from the line sensor camera are converted into digital image data and sequentially, the resultant digital image data is processed under micro-filtering for emphasizing the fine defects. Alternatively, Japanese Unexamined Patent Publication (Kokai) No. 6-323954 discloses a method for detecting light unevenness (e.g. patterns) or dark unevenness (e.g. blots ) In this method, the resultant digital image data is processed by using an unevenness filter, for emphasizing such an unevenness. That is, hitherto, different kinds of inspection methods have been employed in accordance with the kinds of defects to be inspected.
However, it should be noted that the above-mentioned micro-filtering process comprises a step of emphasizing changes in density information of respective pixels (i.e. picture elements) and a sequent step of averaging the respective changes. Then, when a difference between the averaged changes is more than a predetermined threshold value, it is judged that the inspected product contains a microscopic defect.
Thus, according to the judgement of the micro-filtering process, a difference of density value of one pixel is firstly emphasized and thereafter, when the difference is more than the predetermined value, the object to be inspected is judged to have a defect. Therefore, unless the object to be inspected has a certain extent of the difference of density, the surface defect, such as streaks, cannot be detected.
Since, for example, the above-mentioned streak etc. of low contrast, which is narrow in width and long in the transporting direction, has a small density, the resultant difference in density will be too small to exceed the threshold value. Thus, in this case, it is impossible to detect such a streak or the like.
Therefore, it has been concluded that in the micro-filtering process, it is impossible to detect any the defects exhibiting low contrast.
In the meanwhile, the unevenness filtering process comprises a step of dividing the image data as the density information of pixels into lattices consisting of designated number of pixel matrixes lengthwise and breadthwise, a sequent step of emphasizing its unevenness by integrating the density information of each pixel contained in the respective lattices and a further step of calculating a difference between the so-obtained integration value and the other integration value. Then, if the calculated difference is more than a predetermined value, it is judged that the product contains an unevenness as a defect.
That is to say, in the unevenness filtering process, respective densities of the plural pixels are firstly integrated and the difference between the integration values is defined as an unevenness. Therefore, unless a certain extent of area is ensured for inspection, it is impossible to detect such an unevenness, disadvantageously.
Under such a circumstance, it is therefore an object of the present invention to provide a streak inspection method and a streak inspecting apparatus, by which it is possible to detect the streaks with ease, no matter how low of contrast, narrow in width and long in the transporting direction they may be.
The object of the present invention described above can be accomplished by a streak inspection method of inspecting an existence of a streak on a sheet object to be inspected for defect data, the defect data obtained from pixel image data corresponding to a picture taken of the sheet during transportation, the streak inspection method comprising:
a first step of emphasizing changes in density information for each pixel forming the image data;
a second step of adding plural data emphasized at the first step;
a third step of calculating changes among the data added at the second step; and
a fourth step of detecting the streak by comparing the changes calculated at the third step with a predetermined threshold value;
wherein the first step, the second step and the third step are performed in series.
Alternatively, it is also preferable that the first step comprises:
a step of enlarging a width of the defect data by averaging the emphasized changes in density information. The above-mentioned object can be accomplished by providing A streak inspection apparatus for inspecting an existence of a streak on a sheet object to be inspected for defect data, the defect data obtained from pixel image data corresponding to a picture taken of the sheet during transportation, the streak inspection apparatus comprising:
memory means for storing density information for each pixel forming the image data;
a first change calculating means for emphasizing plural changes in the density information for the each pixel forming the image data;
a second change calculating means for dividing the plural changes data into lattices in a manner that a number of data of the sheet object in a direction of the transportation is larger than a number of data of the sheet object in a direction perpendicular to the direction of the transportation and subsequently, calculating changes in density information in each one of the lattices; and
comparing means for comparing the changes obtained by the second change calculating means with a predetermined threshold value;
wherein the first change calculating means is connected in series with the second change calculating means.
In the present invention, preferably, the first change calculating means comprises means for averaging the plural changes.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompany drawing.